A spin-torque nano-oscillator (STNO) is based on an interaction of a spin-polarized current with a magnetic film. The interaction results in two effects, which are giant magnetoresistance (GMR) and spin-transfer torque. FIG. 1 shows a frontwise cross-section of an STNO 10 according to the prior art. The STNO 10 includes a spin polarizing layer 12, a conductive spacer layer 14 over the spin polarizing layer 12, and a magnetic film 16 over the conductive spacer layer 14. Both the spin polarizing layer 12 and the magnetic film 16 are magnetic materials, such as ferromagnetic materials. As such, the spin polarizing layer 12 has a net polarizing magnetic moment 18, which may be based on permanent magnetization of the spin polarizing layer 12 or from external magnetization (not shown) of the spin polarizing layer 12. The magnetic film 16 has small magnetic elements having magnetic moments 20. In the absence of other influences, the magnetic moments 20 may be approximately aligned with a polarizing magnetic field 22, which may be applied to the magnetic film 16 from an external source (not shown).
FIG. 2 shows details of the STNO 10 illustrated in FIG. 1 according to the prior art. During operation, the STNO 10 receives an entering excitation current EDI. The entering excitation current EDI includes electrons, which are used as charge carriers. Each of the charge carriers has a property known as spin, which is a small quantity of angular momentum intrinsic to the charge carrier. The entering excitation current EDI is normally un-polarized, such that orientation of the spin of the charge carriers is random. However, by passing the entering excitation current EDI through the spin polarizing layer 12 and since the spin polarizing layer 12 has the net polarizing magnetic moment 18, the current inside the spin polarizing layer 12 becomes spin-polarized to provide a spin-polarized excitation current SDI to the conductive spacer layer 14. The conductive spacer layer 14 substantially magnetically isolates the magnetic film 16 from the spin polarizing layer 12, such that the net polarizing magnetic moment 18 does not significantly magnetically influence the magnetic moments 20 of the small magnetic elements in the magnetic film 16.
The spin-polarized excitation current SDI is forwarded through the conductive spacer layer 14 into the magnetic film 16. In an STNO, polarization of the charge carriers in the spin-polarized excitation current SDI produces an excitation of the magnetic moments 20 of the small magnetic elements in the magnetic film 16, such that the spin-polarized excitation current SDI causes the magnetic moments 20 to precess. As such, a spin-transfer torque effect is exerted by the spin-polarized excitation current SDI on the magnetic film 16. Specifically, when the polarization of the charge carriers and magnetization of the film are not parallel, a spin-transfer torque will be exerted to alter the direction of the magnetic moments 20 of the small magnetic elements in the magnetic film 16. This spin-transfer torque is propagated to adjacent small magnetic elements in the magnetic film 16, thereby altering the directions of the magnetic moments 20 of the adjacent small magnetic elements in the magnetic film 16. In this regard, the propagation of the spin-transfer torque drives oscillations in the directions of the magnetic moments 20 of the small magnetic elements in the magnetic film 16. These oscillations are called spin waves 24 and propagate out from the spin-polarized excitation current SDI with declining amplitudes 26. The spin-polarized excitation current SDI flows through the magnetic film 16 and exits as an exiting excitation current XDI. The directions of the excitation currents EDI, SDI, XDI illustrated in FIG. 2 are indicative of the direction of electron flow, which is the direction of charge carrier flow.
FIG. 3 illustrates behavior of a magnetic moment 20 of a single small magnetic element in the magnetic film 16 illustrated in FIG. 2 according to the prior art. The magnetic moments 20 of the small magnetic elements in the magnetic film 16 (FIG. 2) collectively have an overall magnetization direction 28, which may be substantially based on the polarizing magnetic field 22 (FIG. 2). However, when the single small magnetic element is subjected to spin-transfer torque, the direction of its magnetic moment 20 is altered, as shown in FIG. 3. The spin-transfer torque imparts a spin 30 to the magnetic moment 20 that traces an orbit around the magnetization direction 28. The orbit may be approximately elliptical in shape. The orbit may be described as an orbit of precession. A spin-torque force 32 of the spin-transfer torque drives the magnetic moment 20 away from the magnetization direction 28 and a damping force 34 associated with the magnetization direction 28 drives the magnetic moment 20 toward the magnetization direction 28. When an average spin-torque force 32 is equal to an average damping force 34, the spin 30 will have an approximate fixed orbit.